SKELETON OF COMMON PERCH. 95 



thirteen (or sometimes fifteen) of these in the first dorsal, and as in all 

 Acanthopteri with two dorsal fins, they are stout stiff spines, hence called 

 entire fin-rays. The first fin-ray of the second dorsal, and the two first 

 of the anal fin are likewise entire, while the remaining rays as well as 

 all the rays of the caudal fin are * soft ' and 'jointed.' The soft rays 

 break up into filaments at their free termination, and these filaments as 

 well as the stem to a certain extent, are jointed. The caudal fin in 

 the Perch appears externally to be equally developed in its dorsal and 

 ventral half. It may be termed ' homocercal.' But, anatomically speaking, 

 the eight or nine long fin-rays which make up the bulk of the fin are 

 all articulated to inferior arches, and the upward-bent termination of the 

 notochord has therefore in reality the chief part of the fin on its ventral 

 edge. The dozen or so small ' accessory ' rays which belong to the dorsal 

 aspect are inconsiderable in size ; and, balanced by similar rays on the 

 ventral aspect, complete the wedge-shaped outline of the fin. The fin is, 

 therefore, from an anatomical point of view, as ' heterocercal' as is the 

 caudal fin of an Elasmobranch in outward appearance as well as 

 anatomically. 



The paired fins are the pectoral and pelvic or ventral, i.e. the fore and 

 hind-limbs respectively. A forked bone, the supra-temporal scale, connects 

 the fore-limb to the skull. It is followed by two thin, plate-like bones, 

 developed in membrane and properly belonging to the skin, the so-called 

 supra-clavicle and clavicle. These two bones lie immediately below the 

 mucous membrane of the hind wall of the branchial cavity. They are 

 probably not represented in Vertebrata above the class Pisces. The two 

 'clavicles' meet in the median line where they are united by ligament. 

 They bear on their inner and posterior face the true shoulder-girdle. This 

 consists of two flat bones, one, the scapula, more dorsal, small, and perforated 

 by a foramen, the other, the coracoid, large and extending nearly to the 

 median line. There is a thin bone, the 'post-clavicle,' which is attached 

 proximally to the clavicle, and hides the scapula when viewed from within. 

 It is superficial and underlies the surface of the depression internal to the 

 pectoral limb. It has been removed on the right side. To the edge of both 

 scapula and coracoid articulates the fore-limb. The small basal cartilages 

 which immediately articulate with the shoulder-girdle cannot be made out 

 in this specimen. The fin contains about ten soft rays. 



The hind-limb lies immediately behind the median union of the pec- 

 toral arch. The basal part of the limb consists of a long triangular bone 

 produced by the contact of a right and left element, which appear to be 

 homologous with the two metapterygia (right and left), i. e. the posterior 

 basalia of the hind-limb in an Elasmobranch. The true pelvis is therefore 

 absent. Teleostei and Ganoidei, with the exception of Polypterus, which has 

 small pelvic cartilages, agree in this respect, and differ from Elasmobranchii, 



